net: wwan: t7xx: Fix napi rx poll issue

When driver handles the napi rx polling requests, the netdev might
have been released by the dellink logic triggered by the disconnect
operation on user plane. However, in the logic of processing skb in
polling, an invalid netdev is still being used, which causes a panic.

BUG: kernel NULL pointer dereference, address: 00000000000000f1
Oops: 0000 [#1] PREEMPT SMP NOPTI
RIP: 0010:dev_gro_receive+0x3a/0x620
[...]
Call Trace:
<IRQ>
? __die_body+0x68/0xb0
? page_fault_oops+0x379/0x3e0
? exc_page_fault+0x4f/0xa0
? asm_exc_page_fault+0x22/0x30
? __pfx_t7xx_ccmni_recv_skb+0x10/0x10 [mtk_t7xx (HASH:1400 7)]
? dev_gro_receive+0x3a/0x620
napi_gro_receive+0xad/0x170
t7xx_ccmni_recv_skb+0x48/0x70 [mtk_t7xx (HASH:1400 7)]
t7xx_dpmaif_napi_rx_poll+0x590/0x800 [mtk_t7xx (HASH:1400 7)]
net_rx_action+0x103/0x470
irq_exit_rcu+0x13a/0x310
sysvec_apic_timer_interrupt+0x56/0x90
</IRQ>

Fixes: 5545b7b9f294 ("net: wwan: t7xx: Add NAPI support")
Signed-off-by: Jinjian Song <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Paolo Abeni <[e-mail]>

drivers/net/wwan/t7xx/t7xx_netdev.c

···
 	ccmni->ctlb = ctlb;
 	ccmni->dev = dev;
 	atomic_set(&ccmni->usage, 0);
-	ctlb->ccmni_inst[if_id] = ccmni;
+	WRITE_ONCE(ctlb->ccmni_inst[if_id], ccmni);
 
 	ret = register_netdevice(dev);
 	if (ret)
···
 	if (WARN_ON(ctlb->ccmni_inst[if_id] != ccmni))
 		return;
 
+	WRITE_ONCE(ctlb->ccmni_inst[if_id], NULL);
 	unregister_netdevice(dev);
 }
 
···
 
 	skb_cb = T7XX_SKB_CB(skb);
 	netif_id = skb_cb->netif_idx;
-	ccmni = ccmni_ctlb->ccmni_inst[netif_id];
+	ccmni = READ_ONCE(ccmni_ctlb->ccmni_inst[netif_id]);
 	if (!ccmni) {
 		dev_kfree_skb(skb);
 		return;
···
 
 static void t7xx_ccmni_queue_tx_irq_notify(struct t7xx_ccmni_ctrl *ctlb, int qno)
 {
-	struct t7xx_ccmni *ccmni = ctlb->ccmni_inst[0];
+	struct t7xx_ccmni *ccmni = READ_ONCE(ctlb->ccmni_inst[0]);
 	struct netdev_queue *net_queue;
 
 	if (netif_running(ccmni->dev) && atomic_read(&ccmni->usage) > 0) {
···
 
 static void t7xx_ccmni_queue_tx_full_notify(struct t7xx_ccmni_ctrl *ctlb, int qno)
 {
-	struct t7xx_ccmni *ccmni = ctlb->ccmni_inst[0];
+	struct t7xx_ccmni *ccmni = READ_ONCE(ctlb->ccmni_inst[0]);
 	struct netdev_queue *net_queue;
 
 	if (atomic_read(&ccmni->usage) > 0) {
···
 	if (ctlb->md_sta != MD_STATE_READY)
 		return;
 
-	if (!ctlb->ccmni_inst[0]) {
+	if (!READ_ONCE(ctlb->ccmni_inst[0])) {
 		dev_warn(&t7xx_dev->pdev->dev, "No netdev registered yet\n");
 		return;
 	}